TY - JOUR
AU1 - Villano, Caren, M.
AU2 - White, Lori, A.
AB - Wound healing and regeneration are complex processes involving cellular differentiation and migration, cell-cell interactions, and coordinated matrix remodeling. Although most animals have the ability to repair an epidermal lesion, complete appendage regeneration is limited to teleost fish and several aquatic amphibians. Limb regeneration in adult mammals is only found in rare cases, such as deer antlers and rabbit ear cartilage (Goss, 1991). Epimorphic regeneration, the process that results in the functional reconstruction of the lost appendage, requires not only the regulation of cell proliferation and migration but also the recapitulation of the limb pattern. This process begins with the formation of a blastema, a mass of heterogeneous mesenchymal-like cells, between the appendage stump and the wounded epidermis (reviewed in Akimenko et al., 2003). Formation of the blastema is necessary for the regenerative process; however, the molecular mechanisms mediating blastema generation are still unclear. Recent data indicate that fibroblast growth factor (FGF) signaling is necessary for blastema formation (Lee et al., 2005; Poss et al., 2000b) and that members of the wnt signaling family are also involved (Poss et al., 2000a). Growth and elongation of the blastema occurs in an organized manner with the cells proximal to the blastema proliferating at a higher rate than those epithelial cells found distal (Santamaria et al., 1996). Several gene families are implicated in cell-cycle regulation in the regenerating tissue, including msxb, FGF, and the homeodomain proteins hoxd11 and hoxd12 (Akimenko et al., 2003). Finally, in order for the appendage regrowth to result in a functional duplicate of the lost limb, the pattern of the limb must be maintained, which is regulated by the expression of a variety of proteins including the transcription factor evx1 (Borday et al., 2001) as well as the coordinated action of sonic hedgehog (shh), bone morphogenic protein 2b (bmp2b), and patched (ptc) (Borday et al., 2001). In this issue of Toxicological Sciences, Andreasen et al. continue investigations into the effect of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on epimorphic regeneration using the zebrafish caudal fin model. Exposure to TCDD and related compounds results in a variety of lesions in mammals, including alterations in liver function and lipid metabolism, weight loss, immune system suppression, endocrine and nervous system dysfunction, as well as severe skin lesions (Mukerjee, 1998). Although the exact mechanism underlying TCDD-mediated pathologies is not completely elucidated, it is accepted that TCDD mediates the majority of these effects through activation of the aryl hydrocarbon receptor (AhR)–signaling pathway. The AhR and its dimerization partner Arnt (AhR nuclear translocator) are members of the basic Helix-Loop-Helix Per-Arnt-Sim domain family of transcription factors that have diverse biological roles ranging from developmental regulation to environmental sensing (Crews and Fan, 1999). Members of the zebrafish AhR pathway have been characterized including two orthologs of mammalian AhR (zfAhR1 and zfAhR2) and two Arnt orthologs (zfArnt1 and zfArnt2). Data indicate that zfAhR2 and zfArnt1 are functional homologs for the mammalian AhR and Arnt (reviewed in Carney et al., 2006). The work reported by Andreason and colleagues may link to the function of TCDD and the AhR pathway in tissue development and morphogenesis in mammalian development. This process requires similar processes as observed in epimorphic regeneration, including changes in cell proliferation, directional migration, matrix metabolism and deposition, and pattern formation. Data from mammalian systems suggest that TCDD affects processes critical to embryonic development. For example, postnatal development of the seminal vesicles, including vesicle branching and differentiation, is reduced in rats exposed to TCDD in utero (Hamm et al., 2000). Further, aberrant mammary development, characterized by decreased tubule branching, is observed in rats exposed to TCDD (Brown and Lamartiniere, 1995). Mice exposed to TCDD in utero develop cleft palate, which is thought to result from insufficient differentiation of the epithelial cells of the palatal shelves (Abbott and Birnbaum, 1989; Takagi et al., 2000). These examples support the hypothesis that TCDD exposure alters similar processes in both aquatic and mammalian species, ultimately resulting in aberrant cell proliferation, differentiation, and migration as well as dysregulated extracellular matrix (ECM) degradation. Two AhR null murine models also show evidence of uncoordinated tissue/matrix remodeling during development (Gonzalez et al., 1995; Schmidt et al., 1996). AhR null animals have smaller livers as a result of massive portosystemic shunting and maintain fetal vascular structures, suggesting that the process by which these early vessels are removed and replaced by the mature vasculature has been disrupted (Lahvis et al., 2000). In addition, AhR null animals show a 50% reduction in estrous-induced terminal end buds in comparison to wild-type animals, suggesting a role for the AhR in mammary development (Hushka et al., 1998). These data suggest that, as in the zebrafish model, the AhR-signaling pathway may play a role in tissue morphogenesis in mammals. Previous work from the Tanguay laboratory demonstrates that TCDD inhibition of regeneration in the zebrafish model requires the AhR-signaling pathway. In the regeneration of the adult tail fin, the AhR pathway was active in both control and TCDD-exposed animals, demonstrated by the expression of zfAhR2, zfArnt2b/c, and cytochrome p450 1A (CYP1A), a TCDD/AhR pathway target gene (Zodrow and Tanguay, 2003). The adult zebrafish studies were limited by the inability to knock down AhR expression using morpholino oligonucleotides. To address this limitation, the larval zebrafish model was used to determine the involvement of the AhR pathway in TCDD inhibition of fin regeneration. Regeneration of the larval caudal fin was inhibited by TCDD, affecting regenerating tissue without affecting normal growth (Mathew et al., 2006). CYP1A protein was detected in the regenerating caudal fins of TCDD-treated larvae, but not controls, demonstrating that the AhR pathway was activated. Further, TCDD-treated larvae injected with zfAhR2 morpholino demonstrate complete fin regeneration, proving that TCDD-inhibited regeneration is mediated through zfAhR2. Knockout and morpholino data indicate that zfArnt1, but not zfArnt2, is the necessary dimerization partner for zfAhR2, and both are involved in the TCDD inhibition of tissue regeneration in zebrafish. In this issue, Andreason et al. uses microarrays to identify specific genes regulated during the regenerative process in response to TCDD. This analysis reveals that TCDD alters the expression of a variety of genes. Not unexpectedly, levels of detoxification genes, such as CYP1A, CYP1B1, and UDPGT, increased following TCDD treatment versus control-amputated fish. However, the expression of genes critical for the regenerative process is also affected. For example, genes involved in retinoic acid (RA) signaling, wound healing, and amino acid metabolism are downregulated in response to TCDD exposure. Repression of genes involved in ECM synthesis and deposition, such as prolyl 4-hydroxylase α1 and 2, which help stabilize collagen cross-links, indicates that TCDD inhibition of the regenerative process involves impairing maturation of the ECM. Conversely, expression of matrix metalloproteinase (MMP)-13 and other enzymes involved in matrix degradation is increased. MMP-13 is a member of the MMP family of endopeptidases that play critical roles in development and tissue remodeling (Brinckerhoff and Matrisian, 2002). Although it was originally thought that the function of these enzymes was limited to degradation of matrix barriers, such as the basement membranes and the interstitial/stromal ECM, recent evidence indicates that MMP proteolytic activity releases growth factors and receptors that mediate cell proliferation and angiogenesis (Egeblad and Werb, 2002). These data are consistent with the findings in mammalian systems. TCDD exposure of normal human keratinocytes and melanoma cells results in increased expression and activity of the MMPs (Murphy et al., 2004; Villano et al., 2006). Further, microarray analysis of human airway epithelial cells identified MMPs as targets of TCDD and the AhR pathway in this cell type as well (Martinez et al., 2002). Perhaps the most significant finding from the results published in this issue of Toxicological Sciences is the identification of a potential control gene for the regenerative process as a target for TCDD. Expression of sox9b, which is increased during normal fin regeneration, is reduced following TCDD exposure at all the time points examined. sox9b is a member of the sox family of transcription factors which are characterized by a sex-determining region Y gene-like high mobility group domain that binds to DNA and alters its conformation (Yan et al., 2002, 2005). The mammalian homolog sox9 is essential for testes determination and cartilage development (Lefebvre and de Crombrugghe, 1998; Lefebvre et al., 1997; Zhao et al., 1997). Zebrafish and related teleost fish have two sox9 genes, sox9a and sox9b, arising from a gene duplication event (Yan et al., 2005). The zebrafish sox9b gene is expressed in the epithelial cells in the developing branchial arches and is a critical transcription factor regulating chondrogenesis. TCDD inhibition of sox9b is followed by a reduction in sox9b-dependent genes, such as collagen 2a1 (col2a1), demonstrating that the TCDD-mediated reduction in sox9b results in inhibition of sox9b target genes. Mammalian sox9 expression is regulated by the RA-signaling pathway (Afonja et al., 2002). Dietary-derived all-trans RA is the main signaling retinoid in the body and is vital for biological functions such as embryogenesis, growth, and differentiation, as well as for vision and reproduction (Dragnev et al., 2000). Activation of the AhR pathway through exposure to TCDD results in significant changes in RA synthesis, catabolism, transport, and excretion (reviewed in Nilsson and Hakansson, 2002). The interaction between the AhR- and RA-signaling pathways results in changes in matrix metabolism, as demonstrated by their synergistic effect on MMP expression in normal human keratinocytes (Murphy et al., 2004). Changes in sox9b expression in the regenerating fin may result from TCDD/AhR-mediated alterations in RA metabolism and signaling. In support of this hypothesis, the expression of genes involved in RA metabolism and signaling was altered by TCDD exposure, including an increase in retinol dehydrogenase 14 and repression of retinol-binding protein 4, short-chain dehydrogenase/reductase 3, and retinoid X receptor α. Further, the zebrafish sox9b-regulatory region contains multiple potential retinoic acid response elements. These data suggest a role for RA signaling in sox9b expression and in TCDD-inhibited tissue regeneration. Overall, the data presented in Andreason et al. make important contributions to understanding the mechanism of TCDD-induced lesions and the function of the AhR pathway in tissue remodeling and regeneration, as well as help dissect the molecular mechanisms that regulate epimorphic regeneration. In addition to identifying specific genes that mediate TCDD inhibition of regeneration, these data also define a novel regulatory pathway, sox9b, as a target for TCDD and the AhR pathway. The sox9 pathway may be of importance not only in the zebrafish fin regeneration pathway but also in the developmental lesions observed in TCDD-exposed rats and mice. Taken together, these studies support the hypothesis that the AhR pathway has a role in tissue remodeling and that the lesions observed following exposure to TCDD result from inappropriate and sustained activation of AhR signaling. Zebrafish caudal fin regeneration may be the ideal model to examine this hypothesis more closely, as it has the advantages of being a whole-animal model, where both regeneration and embryogenesis can be easily observed and genetically manipulated. References Abbott, B. D., and Birnbaum, L. S. ( 1989 ). TCDD alters medial epithelial cell differentiation during palatogenesis. Toxicol. Appl. 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TI - The Aryl Hydrocarbon Receptor–Signaling Pathway and Tissue Remodeling: Insights from the Zebrafish (Danio rerio) Model System
JF - Toxicological Sciences
DO - 10.1093/toxsci/kfj215
DA - 2006-07-01
UR - https://www.deepdyve.com/lp/oxford-university-press/the-aryl-hydrocarbon-receptor-signaling-pathway-and-tissue-remodeling-0bZvjsII35
SP - 1
EP - 4
VL - 92
IS - 1
DP - DeepDyve
ER -